A tiny dot of the planet Venus is seen on the northwest side of the Sun's disc as viewed in Manila, Philippines, on a rare astronomical occasion in this June 8, 2004 file photo. On June 5, 2012, Venus will pass across the face of the sun, producing a silhouette that no one alive today will likely see again. Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it. (BULLIT MARQUEZ/AP)

A tiny dot of the planet Venus is seen on the northwest side of the Sun's disc as viewed in Manila, Philippines, on a rare astronomical occasion in this June 8, 2004 file photo. On June 5, 2012, Venus will pass across the face of the sun, producing a silhouette that no one alive today will likely see again. Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it.(BULLIT MARQUEZ/AP)

It is being billed as one of the rarest astronomical events of the century.

On Tuesday June 5th, the Earth, Venus and the Sun will be in perfect alignment. You will be able to witness the silhouette of Venus slowly moving across the face of the sun – a six-hour journey known as the Transit of Venus.

Venus moves to pass across the sun in this image captured by Japan's satellite Hinode June 6, 2012.
JAXA/Handout

In Pictures

For professional astronomers and amateur sky watchers, it is special treat that’s packed with historical significance and a valuable source of scientific data.

In earlier centuries, observations made during previous transits provided a better understanding of the vast size of the solar system. And now, astronomers are getting ready to take detailed measurements that could aid in the detection of earth-like planets around distant stars.

Because the orbits of the Earth and Venus are slightly tilted, they seldom line up in a straight line with the sun. Transits occur in pairs eight years apart, but the pairs are separated by more than a century. The last transit was in 2004, the first since 1882. When tomorrow’s transit is over, there won’t be another one until 2117.

“So if you ever want to see a Transit of Venus this is your chance,” said Jay Pasachoff, a professor of astronomy at Williams College in Massachusetts.

MAKING USE OF TRANSITS

Although Transits of Venus have occurred for eons, they escaped attention until the 16th century Polish astronomer Nicolaus Copernicus postulated that the sun – and not the earth – is the centre of the solar system.

Based on the motions of the planets in the new sun-centric cosmos, the German astronomer Johannes Kepler predicted there would be a transit of Venus in 1631 – but no one saw it. Kepler was dead by that time and the transit wasn’t actually visible from Europe.

A little-known English astronomer and church cleric, Jeremiah Horrocks, used Kelper’s figures to determine there would be another transit in 1639. Horrocks and a friend, William Crabtree, were the first people to see Venus on the face of the sun.

In 1716, Edmond Halley, the English astronomer best known for predicting the return of the comet that bears his name, proposed that Transits of Venus could be used for settling one of the most challenging scientific questions of his era – the size of the solar system.

If a transit were witnessed from two locations widely separated in latitude, Venus would appear to cut across the Sun on slightly different paths – an optical effect known as parallax. By taking very precise measurements of the time it takes Venus to traverse the Sun, combined with the positions of the observers on Earth, it is possible to calculate the distance between Earth and Venus using geometry.

“You are basically setting up a triangle in space,” explained Roy Bishop, a retired physics professor at Acadia University. Once the distance between Earth and Venus is known, the distances to the other planets can be calculated using Johannes Kepler's third law of planetary motion involving how long the planets take to orbit the Sun.

The next pair of transits would occur in 1761 and 1769. Halley, born in 1656, realized he would not live long enough to witness them. So he urged the next generation to organize scientific expeditions that would gather transit observations from far-flung spots around the globe.

THE FIRST SPACE RACE

The 18th century marked what could be considered the first space race.

The major European powers, at the urging of Halley, financed dozens of scientific expeditions to distant parts of the globe to observe the pair of Venus transits in 1761 and 1769. From these observations, astronomers hoped to gain a better understanding of the size of the solar system.

Beneath this lofty goal, the individual states advanced their own political ambitions. And, in one case, the transit was used as a cover to grab more territory.

Still, many of those who ventured abroad suffered great personal hardship in the pursuit of science.

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